Fix OAuth login by prompting for username if needed (fixes #4108)

[Nutomic]

Testing this locally by temporarily changing the "Authorization callback URL" in Github. Correctly prompts for username now, but in the end it returns "https://voyager.lemmy.ml/api/v4/oauth/authenticate". Server logs show this:

lemmy-1  | 2026-05-21T11:20:05.210051Z DEBUG HTTP request{http.method=POST http.route=/api/v4/oauth/authenticate http.flavor=1.1 http.scheme=http http.host=voyager.lemmy.ml http.client_ip=88.13.133.212 http.user_agent=Mozilla/5.0 (X11; Linux x86_64; rv:140.0) Gecko/20100101 Firefox/140.0 http.target=/api/v4/oauth/authenticate otel.name=POST /api/v4/oauth/authenticate otel.kind="server" request_id=4ff4cfc8-2539-453a-8dd0-a3587af9e84e}: actix_web::middleware::logger: Error in response: LemmyError { message: OauthLoginFailed, caller: crates/utils/src/error.rs:278:20, inner: error decoding response body
lemmy-1  |
lemmy-1  | Caused by:
lemmy-1  |     missing field `access_token` at line 1 column 236
lemmy-1  |
lemmy-1  | Stack backtrace:
lemmy-1  |    0: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
lemmy-1  |    1: <core::result::Result<T,E> as lemmy_utils::error::LemmyErrorExt<T,E>>::with_lemmy_type::{{closure}}
lemmy-1  |    2: lemmy_api_crud::user::create::authenticate_with_oauth::{{closure}}
lemmy-1  |    3: actix_web::handler::handler_service::{{closure}}::{{closure}}

[Nutomic]

We also need to do something similar to ask for registration application if required. Or simply bypass registration application if OAuth is used?

[dessalines]

I think I'd prefer to keep the registration application required. They're not just used to screen for bots, but also for making sure the user is a good fit for the instance.

[Nutomic]

Its working now! You can test by running LEMMY_UI_BACKEND=https://voyager.lemmy.ml pnpm dev and then doing OAuth login with your Github account.